The present invention relates to nonaqueous cells utilizing an active metal anode, a nonaqueous electrolyte solution based on an organic solvent, and a cathode containing an active cathode material such as manganese dioxide, iron sulfide, copper oxide or the equivalent. The development of such high energy battery systems requires the compatibility of an electrolyte possessing desirable electrochemical properties with highly reactive anodic materials, such as lithium, sodium and the like, and the efficient use of active cathode materials such as the manganese dioxide. The use of aqueous electrolytes is precluded in these systems since the anode materials are sufficiently active to react with water chemically. It has therefore been necessary, in order to realize the high energy density obtainable through use of these highly reactive anodes and active cathode materials, to turn to the investigation of nonaqueous electrolyte systems and more particularly to nonaqueous electrolyte systems based on organic solvents.
The term "nonaqueous electrolyte" in the prior art refers to an electrolyte which is composed of a solute, for example, a salt or a complex salt of Group I-A, Group II-A, or Group III-A elements of the Periodic Table, dissolved in an appropriate nonaqueous solvent. Conventional organic solvents include for example propylene carbonate, ethylene carbonate or .gamma.-(gamma)butyrolactone. The term "Periodic Table" as used herein refers to the Periodic Table of the elements as set forth on the inside front cover of the Handbook of Chemistry and Physics, 63rd Edition, CRC Press Inc., Boca Raton, Fla., 1982-1983.
Active cathode materials such as manganese dioxide inherently contain an unacceptable amount of water, both of the adsorbed and bound (absorbed) types, which is sufficient to cause anode corrosion along with its associated hydrogen evolution. This type of corrosion, which causes gas evolution, is a serious problem in sealed cells, particularly in miniature type button cells. In order to maintain battery-powered electronic devices as compact as possible, the electronic devices are usually designed with cavities to accommodate the miniature cells as their power source. The cavities are usually made so that a cell can be snugly positioned therein thus making electronic contact with appropriate terminals within the device. A major potential problem in the use of cell-powered devices of this nature is that, if the gas evolution causes the cell to bulge, the cell could become wedged within the cavity. This could result in damage to the device. Also, if electrolyte leaks from the cell it could cause damage to the device. It is therefore important that the physical dimensions of the cell's housing remain constant during discharge and that the cell will not leak any electrolyte into the device being powered.
In order to reduce the water content in the cathode material, several processes have been developed. For example, U.S. Pat. No. 4,133,856 discloses a process for producing an MnO.sub.2 electrode (cathode) for nonaqueous cells whereby the MnO.sub.2 is initially heated within a range of 350 degrees C. to 430 degrees C. so as to substantially remove both the adsorbed and bound water and then, after being formed into an electrode with a conductive agent and binder, it is further heated in a range of 200 degrees C. to 350 degrees C. prior to its assembly into a cell. British Pat. No. 1,199,426 also discloses the heat treatment of MnO.sub.2 in air at 250 degrees C. to 450 degrees C. to substantially remove its water component.
Although cathode materials with reduced water content are better suited for nonaqueous cell systems, it has been noted that cells employing this type of active material have a tendency to show increased internal impedance during storage. This condition is accompanied by poor closed circuit voltage, poor high and low temperature shelf life, poor cell voltage maintenance characteristics, and poor pulse rate and discharge capabilities.
In an attempt to solve these problems, various additives have been proposed for incorporation into the cathodic material. U.S. Pat. No. 4,465,747 to Evans discloses incorporating additives such as lithium silicate, lithium borate, lithium molybdate, lithium phosphate or lithium tungstate in the cathodic material to suppress the build up of internal impedance in the cell during storage and discharge. It also discloses that alkaline earth metal hydroxides or carbonates can be added to the cathodic material for the same purpose. In U.S. Pat. No. 4,478,921, I have previously suggested the use of manganese carbonate or a combination of manganese carbonate and an alkaline earth metal hydroxide or carbonate for the same purpose.
While the use of these additives tends to improve the cell performance, they seem to be rather specifically oriented. For example, the use of calcium hydroxide seems to improve closed circuit voltage retention after storage at elevated temperatures, e.g., 60 degrees C. However, the lower temperature discharge capacity is not as good as one might desire. On the other hand while carbonates appear to improve low temperature performance, closed circuit voltage maintenance and discharge capacity retention after storage at elevated temperatures is not as great when carbonate additives are used.
In an attempt to get an additive effect as to properties, calcium hydroxide and manganese carbonate were used in combination. However, closed circuit voltage retention after storage wa not thereby improved and indeed was nowhere near that achieved using calcium hydroxide alone.